1. Field of the Invention
The present invention relates to light emitting device drive circuits, and more specifically, to a drive circuit of light emitting devices for use in optical communications with a digital modulation method and a drive circuit for stabilizing light output particularly in the presence of temperature changes, etc.
2. Description of the Background Art
As light emitting devices for use in optical communications, laser diodes and LEDs are generally considered. A direct modulation method of extracting light output in accordance with the amount of current to be injected using these devices has been widely used because it can be easily realized at relatively low costs.
However, these light emitting devices have a characteristic that light output changes with respect to temperature changes. This is especially the case in a laser diode since both of its threshold current and slope efficiency (light emitting efficiency) generally have a temperature change characteristic. Therefore, in the presence of temperature changes, a method for stabilizing light output is generally adapted in order to make light output constant. For example, in a laser diode drive circuit described in Semiconductor Devices for optical communication (Springer-Verlag publishing) p. 184, and a laser diode drive circuit described in Japanese Patent Laid-Open No. 9-64441, in order to hold light output constant, a photodiode for monitoring light output is provided on the back of the laser diode, and on the basis of the output current of the photodiode, feedback control so as to always make light output constant. That is, in these drive circuits, a bias current of the laser diode is feedback-controlled based on a mean value of output of the photodiode for monitoring.
In recent years, as a method for optical communications, the number of cases where a burst signal is handled has increased. However, in the burst signal, its mark rate (the average rate at which the same logic appears) is not constant, and thereby it is impossible to keep light output constant in a method of controlling the bias current by the mean value of output of the photodiode for monitoring. Thus, it is necessary to detect a peak value of output of the photodiode for monitoring. In order to detect the peak value, a relatively high-speed photodiode is required. In the high-speed photodiode, its light receiving area is relatively small, and therefore, in a step of installing the photodiode on the back of the laser diode as a module, it is generally required to align the optical axis, thereby disadvantageously increasing costs of the module and limiting a reduction of the size of the module.
Furthermore, when handling the burst signal during a no signal period, the current consumed in the drive circuit is preferably minimized. In order to achieve this, suggested is a high speed APC circuit described in Japanese Patent Laid-Open No. 7-154016. In the high speed APC circuit, adapted is a method of switching the current injected into the laser diode using an emitter-coupled type circuit. That is, this high speed APC circuit uses an analog switch which is connected to a base of a transistor for use as a current source connected to the emitter-coupled type circuit, and is constructed so as to control the current source based on a current signal from the photodiode for monitoring during a period where a signal exists and to change the analog switch to set base-emitter voltage of the transistor for use as the source to 0 during a no-signal period of the burst signal, thereby minimizing the current consumption.
However, in general, it is difficult to operate the analog switch at a high speed because its response rate is limited. Therefore, in the structure such as that of the above high-speed APC circuit, it is difficult to operate the light emitting device drive circuit accurately from the first bit of the burst signal immediately after switching.